An active matrix liquid crystal display provided with a thin film transistor (TFT) for each pixel electrode of a liquid crystal panel has recently been studied because a quality image higher than that of a simple matrix display device can be obtained.
A conventional active matrix type display device will be explained with reference to the accompanying drawings. FIG. 11 is a perspective view schematically showing a liquid crystal panel part of a conventional active matrix type liquid crystal display device. In FIG. 11, a first substrate 101 of a transparent glass is located opposite to a second substrate 102 of the same transparent glass. On the first substrate 101, there are formed a pixel electrode 103 for driving said liquid crystal in a matrix configuration, a scanning line 104 for applying in a predetermined cycle, a voltage to each pixel electrode 103, a data line 105 for applying an image signal voltage to each pixel electrode 103, and a thin film transistor part 106 comprising a source electrode 106s connected to the data line 105, a gate electrode 106d connected to the scanning line 104 and a drain electrode 106d connected to the pixel electrode 103. This first substrate 101 is generally referred to as an array substrate. On the other hand, on the second substrate 102, there is a transparent conductive film 107 which serves as an opposite electrode of the pixel electrode 103 on the array substrate 101. The second substrate 102 is generally referred to as an opposite substrate 102.
Normally, since a transmission type liquid crystal display device is requested to allow light from a back surface light source to pass through, the pixel electrode 103 must be made of a transparent conductive film. Further, when the liquid crystal panel part is provided with a color display, there must be provided with a color filter in each pixel of the opposite substrate 102.
In the liquid crystal panel part constituted in the aforementioned manner, by changing a voltage applied to a liquid crystal layer in accordance with the image signal during the period in which the thin film transistor part 106 is driven, the transmittance of light from a back surface light source which passes through the liquid crystal panel changes. Therefore, the change in transmittance of light is displayed as an image. Incidentally, as a semiconductor material of the thin film transistor part 106, for example, amorphous silicon (a-Si), polycrystal silicon (p-Si) having high mobility, cadmium selenium (CdSe), etc. may be used.
FIG. 12 is a view showing a construction of a typical three-plate type projection system (liquid crystal projector) using a conventional liquid crystal panel as a light valve for optical switch. An optical path of light emitted from a light source such as a metal halide lamp, etc. is divided for each of three primary colors by using a dichroic mirror (DM) and a reflection mirror (M), and is projected on the light valve (LB) comprising liquid crystal panels for each of a red color (R), a green color (G) and a blue color (B). Then, each color is projected on a screen as an image by switching with the light valve (LB). Normally, a TN (twisted nematic) liquid crystal is used for the display mode of the liquid crystal in the liquid crystal panel.
However, since the light valve comprising the TN liquid crystal cuts about half of incident light with a polarizing plate on the incident side, it is difficult to effectively use light emitted from the lamp. In order to effectively use light from the light source, there has recently been suggested a liquid crystal light valve using a polymer dispersion type liquid crystal, wherein a liquid crystal as a liquid crystal material is dispersed in a polymer to control transmittance and scattering of light at the interface between the polymer and the liquid crystal by means of voltage application, thereby performing optical switching (Asia Display '95, S16-3, p343). According to this display system, it is not necessary to use a polarizing plate and, therefore, it is possible to ensure an output of light with respect to the same input by two or more times as compared with the a display method using the TN liquid crystal.
Furthermore, as one element of important performances required for the light valve to obtain a liquid crystal projector with higher brightness and higher contrast, a high aperture rate is required. The aperture rate refers to a ratio of the part contributing to an actual modulation of light to the size of one pixel. The part which does not contribute to the modulation of light in one pixel includes a thin film transistor part, a scanning line part, a data line part and an auxiliary volume part which runs parallel with respect to a liquid crystal, and a gap part between the pixel electrode and each bus line. In this conventional active matrix liquid crystal display, there are the following problems.
Firstly, when the part which does not contribute to the modulation of light is irradiated with light, for example, when light is allowed to be incident on a channel of the thin film transistor part, there arises a problem in that a current in the OFF state increases, as shown in a characteristic curve of a drain current (Id)--gate voltage (Vg) of the transistor of FIG. 13, so that the switching characteristic is deteriorated. Especially, when using the polymer liquid crystal dispersion type liquid crystal as a liquid crystal material used in the liquid crystal panel, an operation error of the transistor is caused by light scattering. When using a single crystal silicon or a polycrystal silicon having high mobility as the material of the thin film transistor, a coplanar type structure which is present on the side of the glass substrate with respect to the gate electrode is provided as the structure of the thin film transistor. In this case, since the channel region of the thin film transistor is formed in a self-matching manner immediately below the gate electrode, 10 it is required for light to be incident from the side of the opposite substrate because the gate electrode must be used as the light shielding material. When the liquid crystal material to be-used would be the TN liquid crystal, there is no problem because light passing through the panel basically or proceeds in a straight direction. On the other hand, when using the polymer dispersion type liquid crystal as the liquid crystal material, light scattering arises inside of the liquid crystal so that light scattered at the angle shown in the drawings invades the lower part of the gate electrode to increase the off current of the transistor, which results in deterioration of the contrast.
Secondly, in the conventional light valve using the TN liquid crystal, a black matrix for shielding these gap parts was formed on the side of the opposite substrate to prevent light from being incident on the channel part of the thin film transistor part and to cover the region where the liquid crystal is not modulated with the voltage and prevent light leakage in this region, which results in improvement of the contrast. However, the size of the black matrix must be determined by taking the precision of lamination of the array substrate, so that a proportion of the black matrix part to the pixel increases with the decrease of the size of the pixel. Consequently, the aperture rate is lowered. On the other hand, miniaturization of the system requires the liquid crystal light valve to be smaller, but the miniaturization of the liquid crystal light valve can become a limitation factor against the aperture rate. For example, seen from FIG. 13 showing a relation between the panel size and the aperture rate of a conventional general liquid crystal light valve (the assemblage precision: 3 m; the pixel number: 640.times.480; the alignment precision of the array substrate: 2 m; both minimum line width (L.sub.min) and space (S.sub.min) between the same minimum layers: 5 m), the panel size of about 1.5 inch has an upper limit of 55% as the aperture rate.
Therefore, considering a higher aperture rate with respect to the problem of the miniaturization, there has been proposed the BM-on-array technique for moving the aforementioned black matrix (BM) layer from the opposite substrate side to the array substrate side (Display Producing Technology Conference, Santa Clara, 1995, pp 107). As the black matrix on the matrix array according to the BM matrix technique, a photosensitive black resin material is used. However, in this case, when using a black resin as a black matrix material, 1 m or more of the film thickness is required because of the limitation of the light shielding rate of the resin material, so that the non-oriented region of the liquid crystal is formed in the vicinity of the step portion of the edge of the black matrix, resulting in a problem that the display characteristic such as light leakage, etc. is deteriorated. Furthermore, there is an another problem that, the temperature inside of the panel rises by light irradiation because the thermal conductivity of the black resin is low and, resulting in a variation in transmittance caused by the unevenness of the temperature inside of the panel and thus unevenness of the display chacteristics.